In the petrochemical industry, centrifugal compressors may be located at intervals along a natural gas pipeline to boost the gas pressure for processing, to counter the effect of flow losses along the transmission pipelines and to generally keep the gas moving towards its destination. These compressors can be used upstream (during exploration and production), midstream (during processing, storage and transportation), or downstream (during natural gas/and petrochemical refining, transmission and distribution) in a petrochemical process. These centrifugal compressors can also be used to transport other fluids.
To move natural gas or other fluids, centrifugal compressors use a rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, thereby increasing the velocity of the gas. A diffuser (divergent duct) converts the velocity energy to pressure energy.
Dry gas seals may be used to reduce frictional wear on the rotating components while preventing leakage of the centrifuged or processed gas. To further inhibit leakage of processed gas into the atmosphere, some centrifugal compressors can include a pair of dry gas seals working in tandem. One example of such a mechanical seal system is described in U.S. Pat. No. 8,651,801, the contents of which are incorporated by reference herein.
Referring to FIG. 1, a partial cross-sectional view of the tandem non-contacting dry gas seal arrangement 100 of the '801 patent is depicted. At least a portion of the seal arrangement 100 is positioned between a rotating compressor shaft 102 and a compressor housing 104. The rotating compressor shaft 102 is operably coupled to a compressor impeller (not shown) disposed in a process cavity 106 of the compressor, and is supported by the housing 104 via a bearing (not shown) disposed in a bearing cavity 108 of the housing 104. A bore 110 formed in the compressor housing 104 extends between the process cavity 106 and the bearing cavity 108 and defines an annular seal chamber 112. A shroud or labyrinth seal 114, which extends over a radially extending opening formed between the rotating compressor shaft 102 and the compressor housing 104, inhibits the free flow of process gas from the process cavity 106 into the bore 110.
Process gas present in the process cavity 106, which can reach pressures of 6500 psig (450 bar-g), is sealed from the bearing cavity 108 and the atmosphere or surrounding environment by a first stage seal 116 and a second stage seal 118. The first stage seal 116 includes a rotating ring 120 (alternatively referred to as a mating ring) operably coupled to rotating compressor shaft 102 via sleeve 122. Rotating ring 120 defines a radial seal face 124 in relatively rotatable sealing relation with a radial seal face 126 of a non-rotating or stationary ring 128 (alternatively referred to as a primary ring). The stationary ring 128 is operably coupled to the compressor housing 104 via a biasing spring 130 and a spring carrier ring 132, thereby enabling axial movement of the stationary ring 128 relative to the rotating ring 120, so as to enable dimensional changes in the gap width between the rotating and stationary seal faces 124, 126. Generally, the biasing spring 130 in the spring carrier ring 132 biases the stationary ring 128 towards the rotating ring 120. A pressurized gas introduced between the seal faces 124, 126 presents a counteracting force against the biasing spring 130 to increase the gap width between the seal faces 124, 126, so as to enable effective sealing while inhibiting frictional wear of the seal faces 124, 126 to enhance durability. The second stage seal 118 is constructed in a similar manner.
During operation, a flow of diverted process gas referred to as “sealing gas” is provided to the first stage seal 116. The diverted process gas is generally filtered and treated in a gas conditioning unit (not shown), for example by heating and/or drying the process gas to remove vapor particles and liquids, prior to delivery to the first stage seal 116. Once properly conditioned, the sealing gas, which is usually pressurized at or above the pressure of the process gas in the process cavity 106, flows through an inlet 134, into a chamber 136, through the first stage seal 116, into a chamber 138, and out through an outlet 140. Additionally, due to the pressure differential, a portion of the sealing gas typically flows past the labyrinth seal 114 and into the process cavity 106, thus creating a gas flow in a direction that prevents unfiltered and untreated process gas from entering the annular seal chamber 112.
In a similar manner, a “barrier gas,” which is typically an inert gas such as nitrogen (N2), is provided to the second stage seal 118. The barrier gas, which is usually pressurized to a pressure slightly higher than the pressure of the gases in the chamber 138, flows through an inlet 142, into a chamber 144, through second stage seal 118, into a chamber 146, and out through an outlet 148. Additionally, a portion of the barrier gas flows from the chamber 144 to the chamber 138, and out through the outlet 140 with the sealing gas.
In some seal arrangements, an additional gas seal 150, referred to as a “separation gas seal” can be configured to isolate the annular seal chamber 112 from oil within the bearing chamber 108. The separation gas flows through inlet 152, through the separation gas seal 150, into a chamber 146, and out through an outlet 148.
The portion of the flow of the filtered and treated process gas (sealing gas), which in some cases is natural gas consisting mostly of methane, exits through the outlet 140 and often a portion of it is ported directly to the atmosphere. This leakage, which is typically around eight standard cubic feet per minute (SCFM) per centrifugal compressor, has been considered by the industry to be an acceptable amount.
Methane (CH4) is a greenhouse gas that has been shown to adversely affect climate change. Recent years have seen rising levels of methane emissions to the atmosphere due to increasing availability of natural gas resources coupled with aging natural gas distribution systems. Presently, it is estimated that 20% of methane leaks occur within natural gas distribution systems. Centrifugal compressor leakage is one of the largest sources of methane emissions in natural gas distribution systems. According to some estimates, compressor emissions account for roughly 500 metric tons of methane leakage per annum per facility.
A system that further reduces the emission of natural gas or other process gases to the atmosphere, without requiring a costly replacement of numerous centrifugal compressor components, would provide a distinct advantage over conventional systems presently used in the natural gas industry.